May's government said her permanent envoy to the EU had informed European Council President Donald Tusk of the date when Britain intends to invoke Article 50 of its Lisbon Treaty - the mechanism for starting its exit after a referendum last June in which Britons voted by a 52-48 percent margin to leave the bloc.

Britain's Prime Minister Theresa May signs a Swansea City deal during a meeting at the Liberty Stadium in Swansea March 20, 2017. REUTERS/Ben Birchall/Pool

The EU said it was ready to begin the negotiations and within 48 hours of the trigger on March 29, Tusk will send the other 27 member states his draft negotiating guidelines, which means that talks could start in May.

Sterling fell half a cent against the dollar on what Brexit minister David Davis described as a move taking Britain to "the threshold of the most important negotiation for this country for a generation".

"The government is clear in its aims: a deal that works for every nation and region of the UK and indeed for all of Europe – a new, positive partnership between the UK and our friends and allies in the European Union."

May, 60, hopes to negotiate terms that keep trade, financial and political relations with EU member states as close as possible after Brexit, but also satisfy euroskeptics in her Conservative Party who demand a complete break from an institution they say has stolen British sovereignty.

It will be a difficult and ambitious balancing act. Talks on departing the prosperous club Britain joined in 1973 are likely to be the most complex London has held since World War Two, with other EU leaders saying they will not give May an easy ride.

With nationalism and anti-establishment, anti-immigrant sentiment spreading across Western Europe, the EU leadership in Brussels is anxious to avoid encouraging others in the 28-member bloc to bolt.

At the same time, May faces threats by Scottish nationalists to call a new independence referendum that could break up the United Kingdom and fears in Northern Ireland that a "hard border" with EU member Ireland will return after Brexit.

May has revealed little of her strategy for securing what she calls "the best possible deal" for the world's fifth largest economy and making Brexit as painless as possible.